This invention relates to an improved sensor for measuring deceleration of a road wheel on a vehicle. The sensor is especially useful in a control mechanism for a vehicle anti-skid brake system.
In some anti-skid brake systems braking torque is applied until vehicle deceleration exceeds a predetermined threshold, at which point the braking force is automatically removed or lessened; the deceleration threshold is selected to prevent wheel lock-up and incipient skid condition. When the deceleration rate is sufficiently lessened below the skid threshold level the braking force is reapplied. Overall braking action is a cyclic pulse type application of the brake force at frequencies greater than five or ten braking pulses per second. The system requires a control system that includes a road wheel deceleration sensor having a relatively quick response to the parameter being sensed. The present invention is directed to a sensor having the required quick response.
The preferred sensor is designed to respond to wheel deceleration rather than absolute wheel speed. It has been determined that braking is most effective at the point where the rate of change of deceleration is zero. At this point the rate of change of the coefficient of friction of the tire on the road is a maximum so that braking then has its greatest effectiveness. An effective braking system can be designed around a wheel sensor responsive to wheel deceleration rather than wheel speed. The system envisioned by applicant includes a fluidic amplifier and control system wherein sensed values of deceleration are fluidically differentiated to obtain the rate of change of deceleration. The resulting signal and a second signal representing the desired limit on rate of change of the deceleration signal are applied to a fluidic oscillator to provide an amplified control signal for the brake pressure. As previously noted, the present invention relates to an improved sensor responsive to road wheel deceleration forces.